Zeolites are crystalline or quasi-crystalline aluminosilicates constructed of repeating SiO4 and AlO4 tetrahedral units. These units are linked together to form frameworks having regular intra-crystalline cavities and channels of molecular dimensions. Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement of its tetrahedral units. By convention, each framework type is assigned a unique three-letter code (e.g., “AEI”) by the International Zeolite Association (IZA).
Synthetic zeolites are typically produced using a structure directing agent (SDA), also referred to as a “template” or “templating agent”. SDAs are typically complex organic molecules which guide or direct the molecular shape and pattern of the zeolite's framework. Generally, the SDA serves to position hydrated silica and alumina and/or as a mold around which the zeolite crystals form. After the crystals are formed, the SDA is removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
Zeolites have numerous industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas may be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst.
ZSM-5 and Beta zeolites have been studied as SCR catalysts due to their relatively wide temperature activity window. However, the relatively large pore structures of these zeolites have a number of drawbacks. First, they are susceptible to high temperature hydrothermal degradation resulting in a loss of activity. Also, large and medium pore sizes tend to adsorb hydrocarbons which are oxidized as the temperature of the catalyst increases, thus generating a significant exotherm which can thermally damage the catalyst. This problem is particularly acute in lean-burn systems, such as vehicular diesel engines, where significant quantities of hydrocarbon can be adsorbed during cold-start. Coking by hydrocarbons presents another significant drawback of these relatively large and medium pore molecular sieve catalysts. In contrast, small pore molecular sieve materials, such as those having an AEI framework, offer an improvement in that fewer hydrocarbons are able to permeate into the framework.
To promote the catalytic reaction, transition metals may be included in the zeolite material, either as a substituted framework metal (commonly referred to as “metal-substituted zeolite”) or as a post-synthesis ion exchanged or impregnated metal (commonly referred to as “metal-exchanged zeolite”). As used herein, the term “post-synthesis” means subsequent to zeolite crystallization. The typical process for incorporating a transition metal into a zeolite is by cationic exchange or impregnation of metals or precursors after the molecular sieve is formed. However, these exchange and impregnation processes for incorporating metals frequently lead to poor uniformity of metal distribution, particularly when exchanged into small pore molecular sieve.